1. Field of the Invention
The present invention relates generally to nuclear quadrupole resonance (NQR) and more specifically to the detection of explosives and narcotics by nuclear quadrupole resonance.
2. Description of the Background Art
As described in U.S. Ser. No. 07/704,744, filed May 23, 1991, now U.S. Pat. No. 5,206,592 and in U.S. Ser. No. 07/730,772, filed Jul. 16, 1991, now U.S. Pat. No. 5,238,234 both of which are incorporated herein by reference, NQR can be an effective means of detecting explosives and narcotics. In particular, NQR is useful in the detection of nitrogenous or chlorine-containing explosives and narcotics (or, more generally, materials containing quadrupolar nuclei such as .sup.14 N, .sup.35,37 Cl, etc), such as carried in luggage, mail, small cargo or on a person. This general NQR approach is referred to as `pure` NQR to indicate that no externally applied static magnetic field is required.
Unfortunately, the radiofrequency (RF) pulses used in the typical NQR explosives and narcotics detection sequences will induce an acoustic ringing in certain items, typically magnetized iron or ceramic, which may occasionally be found in baggage. This ringing may last on the order of a millisecond and be comparable in size to the NQR signal amplitude corresponding to a threat quantity of explosives.
If this acoustic ringing were not removed, then one could suffer an increased false alarm rate (false positive) for those bags containing such materials. Alternatively, a low false alarm rate could be achieved by increasing substantially the threshold `alarm` setting, but at the expense of increasing the minimum detectable quantity.
Acoustic ringing (sometimes called magnetoacoustic ringing or probe ringing) is a well-known, though not completely well-understood, phenomenon in conventional NMR, where an external, static magnetic field is used. The basic mechanism is that the RF pulse induces eddy currents in a conductor. In a magnetic field, a force acts on these currents and hence on the conductor, inducing acoustical energy that bounces back and forth within the conductor. This pulse of acoustic energy correspondingly alters the magnetic coupling to the receiver coil, inducing a `signal` that is in phase with the driving RF pulse and that persists until the acoustic energy is dissipated in the system.
Since there is no static magnetic field in pure NQR, the acoustic ringing mechanism is different than for NMR applications. A likely explanation is that the ferromagnetic domains in magnetized materials try to realign in response to the applied RF magnetic field. These (partial) realignments cause lattice distortions generating acoustic energy that then reflects back and forth within the sample.
For concreteness the present invention disclosure considers the elimination of acoustic ringing, though it will be appreciated that other mechanisms that contribute to extraneous probe ringing are also amenable to elimination according to the approach described herein.
In the related art of nuclear magnetic resonance (NMR), there are a number of approaches to reducing or removing extraneous probe ringing. Since the ringing is generally in the probe body or RF coil, rather than the specimen under study, mechanically redesigning the probe to employ materials that rapidly damp the acoustic wave is a viable option in NMR.
There are also a number of NMR pulse sequences that have proven effective in largely eliminating the effects of acoustic ringing. Such sequences generally rely upon the ability to invert the sign of the NMR signal in the rotating reference frame, but not that of the acoustic ringing signal in the same frame (or vice-versa). Thus, provided the offending acoustic ringing signal is reproducible over time, one can arrange to alternatively add and subtract the incoming signal so that the desired NMR signal is consistently added (in effect), while the acoustic signal alternately adds and subtracts to zero. As will be shown below, the cancellation techniques conventionally used for NMR are not directly applicable to NQR.
To remove the acoustic ringing in NMR, a second, well-known approach is to rely on the ability to invert (change the sign of) NMR magnetization by a conventional 180.degree. or .pi. pulse. One possible sequence consists of a .pi./2 excitation pulse that produces, say, a positive NMR signal and a positive acoustic ringing signature. After magnetization is regenerated in a time T.sub.1, the spin-lattice relaxation time, a .pi. inverting pulse is applied, followed a time t.sub.d later by a .pi./2 pulse. All pulses have the same phase. Provided t.sub.d is much less than T.sub.1, the NMR signal will be inverted. Also, provided t.sub.d is long compared to the acoustic ringing signal, the acoustic signal will be the same as it was after the initial .pi./2 pulse. Addition and subtraction of the resulting signals removes the component of acoustic ringing, while preserving the NMR signal.
However, for the general NQR case, these simple strategies presented above are not appropriate. In the detection of explosives or narcotics in a package or on a person by NQR, the acoustic ringing arises from magnetized components within the specimen, e.g. a suitcase or package. Identifying and then removing the offending contents is not a desirable solution. Furthermore, the straightforward sequence discussed above for NMR does not work for NQR. For a polycrystalline specimen, it is well-known that there is no RF pulse that inverts the entire NQR `magnetization`.
Additionally, it is well-known that the exact NQR resonance frequency varies with temperature. Obviously, this temperature variation has some undesirable effects in NQR detection schemes. While conventional schemes to minimize these undesirable effects exist, the method of the present invention can effectively remove both probe ringing and minimize temperature effects.